1. Field of the Invention
The present invention relates to a heart stimulator, particularly a dual chamber pacemaker with an evoked response detector.
2. Description of the Prior Art
For certain conditions such as hypertrophic obstructive cardiomyopathy (HOCM) the patient's condition may improve if he or she is paced 100% in the ventricle. In a state of HOCM the left ventricular wall is asymmetrically thickened. The interventricular septum thickness significantly exceeds that of the opposing posterolateral wall. A pressure gradient exists across the left ventricular outflow tract and during ventricular contraction, a progressive degree of outflow tract obstruction results. The conventional site of ventricular pacing is within the right ventricular apex and pacing, prior to intrinsic R-wave excitation, from this site can favorably alter the degree of obstruction. This has been clinically verified.
100% pacing in the ventricle requires understanding of a phenomenon referred to as fusion. Fusion means that the natural conduction time, which is the time interval between an atrial activity (a sensed P-wave or a delivered A-pulse) and the subsequent natural ventricular activity (R-wave), is the same as the time (AV-interval) between an atrial activity (again, a sensed P-wave or a delivered A-pulse) and the delivery of a ventricular stimulation pulse (V-pulse). Fusion is thus a condition where the V-pulse is delivered at the same time as the R-wave occurs. Thus, fusion means that the V-pulse occurs when the heart tissue is not capable of responding, i.e. it is refractory, a tissue refractory period starting at the depolarization event (R-wave) and remaining until repolarization (T-wave) occurs. Although not necessarily harmful to the heart, fusion causes loss of energy in the V-pulse, and should therefore be avoided to save pacemaker battery energy. In the discussion herein, both the time interval between a P-wave or an A-pulse, and a V-pulse will be referred to as the AV-interval.
To obtain 100% pacing beats with no fusion, very short AV delays have been used. Such very short AV intervals are, however, non-physiologic and therefore it is highly desirable to prolong the AV interval while maintaining a continuous monitoring of fusion, such that the AV interval would be shortened automatically if fusion beats appear. Several attempts have been made to solve this problem.
Thus, U.S. Pat. Nos. 5,534,016 and 5,713,930 describe techniques for optimizing the AV interval for therapeutic purposes for patients having HOCM. In the system according to U.S. Pat. No. 5,534,016 the T-wave detection is monitored to detect when the AV interval is lengthened to the point of evoking a fusion beat, and in the system disclosed in U.S. Pat. No. 5,713,930 the relationship between AV intervals and OT intervals (=the time interval between a delivered ventricular stimulus and resulting T-wave) is monitored and therefrom it is determined when AV intervals correspond to full capture and when AV intervals correspond to fusion.
Further, in U.S. Pat. No. 5,507,782 a dual chamber pacemaker is described in which the longest AV interval which results in complete ventricular capture is determined from the wave form of the ventricular depolarization R-wave following a ventricular pacing pulse for the purpose of treating patients suffering from HOCM. In this document the problems related to fusion beats and the transition region between complete pacing and fusion are not at all dealt with.
Another way of solving the problem of fusion and providing a 100% pacing of the ventricle is by AV node ablation. AV node ablation is, however, an intervention associated with extra costs and the conduction pathway from the atria to the ventricles is then permanently destroyed so the patient will be completely dependent on a pacemaker in the future with higher clinical risks in the event of a pacemaker failure.